Formation And Control Of Multi-Scale Crystalline Structure In POM/PEO Crystalline/Crystalline Blends With High Performances

Polyoxymethylene (POM) is one of the important engineering plastics. Its modification is in great demand for the national economy. In this thesis, poly oxide ethylene (PEO) was adopted to modify POM, and a novel POM/PEO crystalline/crystalline system was prepared and studied. By controlling the multi-scale crystalline structure through adjusting the content and structure of the components, as well as the processing temperature and shear stress, POM/PEO blend with enhanced mechanical properties was obtained, which contributed to the present polymer crystalline/crystalline theory.In this thesis, compatibility of POM/PEO blend, particularly their compatibility induced by the space characteristic of the chain conformation of both PEO and POM molecules was discussed. POM/PEO blends with high impact strength, self-lubrication and anti-wear properties were prepared by controlling the multi-scale crystalline structure in POM/PEO blends through changing the content of components, the temperature and stress during processing, which provides a new way to modify POM with enhanced performances.The main conclusions of the research are as follows:1. Due to their special chain conformations, POM and PEO chain segments can include or tangle each other, endowing POM/PEO with pretty good compatibility at the amorphous state.2. PEO content and molecular weight have great influence on the crystalline structure in POM/PEO blends. PEO with low content or small molecular weight does not crystallize due to the restriction of the confined environment, only exists as amorphous state in the space among POM lamellae, thus enlarging the spherulite size of POM. When increasing PEO content or molecular weight, although the confined environment is extended and PEO can crystallize, the crystallization of PEO is still retrained and crystallization temperature decreases to 5℃, 35℃lower than the crystallization temperature of neat PEO, forming POM/PEO interfibrillar segregation. With further increase of PEO content or molecular weight, PEO crystallizes independently, forming POM/PEO interspherilute segregation.3. In POM/PEO blends, PEO doesn't change the nucleating way of POM but restrains its nucleating rate and crystal growing, resulting in that the crystallization temperature, melting point and relative crystallinity of POM decrease. However, the nucleating way of PEO in the blends changes. POM crystal acts as nucleating agent for PEO, inducing that the crystallization temperature of PEO increases, and melting point and relative crystallinity of PEO decrease.4. The toughness of POM/PEO blend depends on the crystalline structure of the blend. The materials with the crystalline structure that amorphous PEO exists among POM lamellae have the best toughness. However, POM/PEO interfibrillar segregation and the interspherilute segregation crystalline structures seem unfavourable for the toughness of the blends.5. PEO can improve the friction and wear properties of POM/PEO blends because PEO in the blend melts during friction and transfers to the friction surface, forming a lubricative layer. With PEO content increasing, the friction coefficient decreases greatly. The materials with the crystalline structure that amorphous PEO exists among POM lamellae have a less abrasion.6. The way of temperature decreasing in sample preparation has great influence on the crystalline structures and therefore mechanical properties of POM/PEO blends. The size of POM amorphous region and the crystallization of PEO in the process of temperature decrease are the main factors determining the multi-scale crystalline structures in POM/PEO blends: amorphous PEO among POM lamellae, POM/PEO interfibrillar segregation crystalline structure or POM/PEO interspherilute segregation crystalline structure. By model 1 (directly quenching samples from the melt to the temperature below PEO crystallization point), amorphous PEO exiting among POM lamellae or POM/PEO interfibrillar segregation are formed, and the material has pretty good toughness.By model 2 (quenching samples from the melt to PEO crystallization temperature, getting across POM crystallization temperature and allowing PEO to well crystallize), POM/PEO interspherilute segregation crystalline structure in all blends is formed, and the toughness of the materials become worse.By model 3 (decreasing temperature from the melt to POM crystallization temperature and allowing POM to well crystallize, then quenching to the temperature below PEO crystallization point ), When PEO contents are 5% and 10%, interfibrillar segregation crystalline structure is formed, and the blends have higher notched impact strength. But when PEO content is 50%, the crystalline structure of the blend is interspherilute segregation. The blend has better toughness due to the formation of internet phase morphology, but the tensile decreases greatly.By model 4 (letting both PEO and POM well crystallize) and model 5 (decreasing temperature at a rate of 10°C/min), when PEO content is 5%, the blend forms amorphous PEO existing among POM lamellae. When PEO content is 10%, the blend forms interfibrillar segregation crystalline structure. However, when PEO content is 50%, the blend forms interspherilute segregation crystalline structure.In all the decreasing temperature models, when PEO content is 5%, the material prepared by models 4 has the best comprehensive mechanical properties because the crystalline structure of amorphous PEO existing among POM lamellae is formed, and POM also has the highest crystallinity.7. The melt viscosity of POM/PEO blend is sensitive to shear rate. The crystalline structure of POM/PEO blend can be controlled by adjusting the injection speed and the size of dispersing phase in injection process. When PEO content is 5%, with the shear rate increasing, although the dispersing phase of PEO becomes smaller, the crystalline structure that amorphous PEO exists among POM lamellae doesn't change. When PEO content is 10% and 20%, with the shear rate increasing, the crystalline structure of POM/PEO blend changes from interspherilute segregation to interfibrillar segregation, resulting in an increase of notched impact strength.8. The size of PEO dispersing phase affects the crystalline structure in POM/PEO blend. There may be a critical value of PEO dispersing phase size around 1μm, lager than which forming POM/PEO interspherilute segregation crystalline structure, smaller than which forming POM/PEO interfibrillar segregation crystalline structure.9. When PEO content is 5% and molecular weight is 5.0×10~5, the blend has pretty good comprehensive mechanical properties. The notched impact strength can reach 12.9 kJ/m~2, double that of POM, and the wear scar width and the friction coefficient are 0.18 and 3.5mm respectively, only about 50% and 35% of that of POM.